1. Field of the Invention
The present invention relates to a process and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures, and more particularly, but not by way of limitation, to an improved process and apparatus for recovering vaporized gasoline light ends and the like from a mixture thereof with air expelled from tank trucks and the like.
2. Description of the Prior Art
In handling multicomponent and single component hydrocarbon liquids such as gasoline, kerosene, benzene and the like, air-hydrocarbon vapor mixtures are readily produced which cannot be vented directly to the atmosphere due to the resulting pollution of the environment and fire and/or explosion hazard. Consequently, a variety of processes and apparatus have been developed and used for removing hydrocarbon vapors from such air-hydrocarbon vapor mixtures whereby the remaining air can be safely vented to the atmosphere. The removed hydrocarbons are generally liquefied and recombined with the hydrocarbon liquid from which they were vaporized thereby making their recovery economically advantageous.
A process for the recovery of light mixed hydrocarbon vapors from an air-hydrocarbon mixture expelled as a result of storage breathing or loading of a vented hydrocarbon vessel is described in U.S. Pat. No. 4,066,423. In accordance with such process, the air-hydrocarbon vapor mixture from which hydrocarbons are to be removed and recovered is passed through a bed of solid adsorbent having an affinity for hydrocarbons. As the mixture passes through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed and a residue gas stream is produced which is comprised of substantially hydrocarbon-free air. While a first bed of solid adsorbent is adsorbing hydrocarbons from the mixture, a second bed of solid adsorbent having hydrocarbons adsorbed thereon is regenerated by evacuation. The completeness of the regeneration of the solid adsorption beds of the process is dependent solely on the degree of vacuum produced in the beds by the vacuum pump utilized. Because vacuum pumps are incapable of achieving total vacuum, i.e., lowering the absolute pressure exerted on the beds to zero, a quantity of hydrocarbons are left adsorbed on the beds after regeneration which reduces the capacity of the beds to adsorb additional hydrocarbons and reduces the service life of the adsorbent.
The hydrocarbon-rich air-hydrocarbon mixture produced as a result of the regeneration of the bed is contacted with a liquid absorbent whereby hydrocarbons are removed therefrom and the residue gas stream from the absorption step is recycled to the bed through which the inlet air-hydrocarbon mixture is flowing. In accordance with the teachings of U.S. Pat. No. 4,066,423, the liquid absorbent utilized is liquid hydrocarbons condensed from the air-hydrocarbon vapor mixture produced in the evacuation regeneration step. More specifically, the hydrocarbon-rich air-hydrocarbon vapor mixture is cooled whereby portions of the hydrocarbons are condensed and such condensed hydrocarbons are circulated into contact with the remaining air-hydrocarbon vapor mixture whereby hydrocarbon vapors are absorbed by the liquids.
Numerous other processes and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures or otherwise treating said mixtures have been developed and used heretofore. In all of the prior processes which utilize solid adsorbent for removing hydrocarbons from air-hydrocarbon vapor mixtures, regeneration of the adsorbent is incomplete whereby hydrocarbons are left on the adsorbent reducing the capacity, efficiency and service life thereof.
As is well understood by those skilled in the art, all of the prior art processes described above as well as the process of the present invention can be utilized in applications where the hydrocarbons to be recovered are mixed with inert gases other than air, e.g., nitrogen. Therefore, it is to be understood that while for convenience, the inert gas-hydrocarbon mixtures described in connection with the present invention are referred to as air-hydrocarbon vapor mixtures, the present invention is equally applicable to other inert gas-hydrocarbon vapor mixtures.
As various government regulatory agencies have enacted and enforced hydrocarbon vapor control emission regulations applicable to petroleum product transfer operations, it has been recognized that it is appropriate and fair to state the regulations in a form that relates the allowable level of hydrocarbon emissions into the atmosphere to the volume of liquid transferred. Beginning in the late 1970's and continuing until August, 1983, the United States Environmental Protection Agency (EPA) hydrocarbon vapor emission standard for gasoline bulk terminals was to allow 80 milligrams (mg) of hydrocarbon vapor to be emitted per liter (1) of gasoline product transferred. In August, 1983, the EPA revised the emission standard to limit the allowable hydrocarbon vapor emissions from a gasoline bulk terminal to 35 milligrams of hydrocarbon vapor per liter of gasoline product transferred. Since then, various air quality regulatory agencies have enacted even more stringent regulations involving transfer operations of volatile hydrocarbon liquids. For example, current regulations limit hydrocarbon vapor emissions from gasoline bulk terminals in some regions of California to 10 milligrams per liter. Current regulations applicable for marine transfer operations involving gasoline and crude oil in the San Francisco Bay area of California, limit the allowable hydrocarbon vapor emission level to 5.7 milligram per liter of liquid transferred. Furthermore, it is anticipated that these allowable emission standards will continue to become more and more stringent. It is expected that many countries in Europe will be required to comply with the proposed German standard which will only allow 0.15 milligrams of hydrocarbon vapor to be emitted per liter of vent gas.
The prior art carbon adsorption/absorption processes and systems which are based on the use of liquid ring vacuum pumps combined with air purge for carbon bed regeneration, have been shown to be capable of complying with allowable emission regulations of 10 mg/1 or higher. Under low capacity utilization, they have demonstrated emission performance in the 1 to 10 mg/1 range. However, the latter type of performance represents unusual circumstances which can not be obtained consistently.
The prior art carbon adsorption/absorption processes and systems presently being used are limited by the liquid ring vacuum pumps utilized to the level of vacuum that can be obtained thereby to regenerate the adsorbent beds. Generally this vacuum level is limited by the vapor pressure of the liquid seal fluid required to make the vacuum pump work. Practical vacuum levels that can be achieved by this type of vacuum pump correspond to absolute pressure levels of between 50 and 100 millimeters (mm) of mercury (Hg). This limitation of vacuum limits the degree to which the carbon beds can be regenerated and therefore limits the hydrocarbon vapor recovery efficiency.
By the present invention, an improved process and apparatus is provided which allows significantly greater carbon bed regeneration vacuum levels to be achieved, i.e., vacuum levels equivalent to absolute pressure levels of 1 mm Hg or less. This greater vacuum level combined with high vacuum air purge achieves greater carbon bed regeneration and, as a result, significantly lower hydrocarbon vapor emission levels are obtained, i.e., levels equal to or lower than even the most stringent regulation (German) of 0.15 mg/1 proposed for gasoline transfer operations.